The present invention relates to process control in general, and more particularly, to a decoupling technique implementing energy management and/or optimization in fluid distribution for plant consumption.
Therefore, the invention relates to fluid flow distribution systems in general. More particular, the invention involves the controlled distribution of steam from various pressure lines for energy management and/or for steam-power cogeneration in an industrial plant.
In a steam generating plant, steam of selected quality must be distributed so as to satisfy the demand of an industrial plant and in such a way as to save energy and reduce costs. This is in particular the case where turbogenerators are used to concurrently generated electrical power and deliver steam after expansion through the turbine. Then, energy management in distributing steam from boilers, pressure lines and/or turbine extractions is performed, together with optimization in distributing steam and power from the turbogenerators.
Turbine control optimization in steam-power cogeneration is illustrated in copending patent application Ser. No. 550,164.
As a result of control decisions either under energy management or under the line control optimization, control is effected to change fluid flow on selected lines of distribution. Such control, however, interacts through the process, so that control devices which should not be affected will assume different settings and respond adversely by attenuating the intended flow changes. In order to overcome the effects of this interaction, the present invention provides for anticipating such attenuating effects on the basis of the control decisions and to modify control to the extent of the anticipation, thereby to compensate for the interaction simultaneously when effecting control. This approach has been disclosed as "decoupling" in copending patent application Ser. No. 367,830, filed Apr. 12, 1982, now U.S. Pat. No. 4,500,950, under the title "Industrial Process Control Apparatus and Method". As explained there, non-linear control of a multi-unit industrial combustion process may lead to instability due to the interaction of the various units when one of them is being controlled. Decoupling is implemented with a computer treating the inherent interactions with an algorithm based on a set of data gathered between the different units and processed toward a solution applied to the several units when actually passed into control form. Such decoupling concept has been recognized in the aforementioned application as useful in steam turbine-generator control and in energy management systems where control involves extraction valves, governors and reducing valves of the several units in the system. It was observed, then, that changes made to the control device associated with any one generator, if made on a serial basis, will cause a response from another generator due to natural feedback. It was proposed to decouple the control devices from one another in order to assess the effect that the desired change will also have on the others. Having ascertained the correlative changes induced in the others through the use of a microcomputer, the intended change on the particular control device is implemented simultaneously with such correlative changes on all the other devices. Decoupling, there, was applied to damper positioning as part of automatic control of an exhaust stack of a combustion chamber. Decoupling is required, there, to overcome the non-linearity introduced by natural draft in the relationship between the combustion chamber pressure and stack damper position. Decoupling control in effect adds a corrective change to the output of the position controller of each damper member and generates the correct control signal for damper positioning. Moreover, decoupling was, there, exercised by a DDC system running.
The stability problem is compounded with a cogeneration controller such as described in the aforementioned copending patent application Ser. No. 550,164 in that optimization of a multi-unit turbogenerator plant is, in that particular case, effected with mass flow balancing on each unit and on the overall steam flow system, together with cogeneration of electrical power while supplying with the same source of steam a plant demand in steam and cogenerated electrical power. In such a context, the interaction between make-up reducing valves and extraction steam flows, or between independently controlled extraction valves discharging into a common steam header, will cause, through the governor links, instability to a large degree. Where an optimizing steam flow/power distribution has been determined as stated in the aforementioned pending application, changing the setting of only one turbogenerator unit at a time, in a sequence, requires numerous executions in sequence before a new stable and optimum distribution is reached for all flows and all units.